1. Field of the Invention
This invention relates to a method for tuning optical characteristic of an optical module.
2. Related Art
An optical filter has the distribution characteristic in the surface. This is generated by manufacturing an optical filter, an internal stress, in the principle etc. Therefore, an optical filter has the characteristic that the center wavelength of the transmission spectrum is different according to an incidence position of optical signal on the filter surface. Moreover, an optical filter has the angle dependency to the incident angle of optical signal.
A conventional optical module is composed of an optical fiber for input/output of optical signal, a lens for optically coupling the input fiber with output optical fiber, and an optical filter for multiplexing/de-multiplexing.
The edge surface of the optical fiber forms the slope. The slope surface is to control the reflection attenuation on the optical fiber edge surface. In a conventional optical module, the optical signal is exited from the optical fiber according to this slope surface of the optical fiber with a predetermined angle inclined to an optical axis based on Snell""s law. The optical signal exited from the optical fiber is inputted the lens from the optical fiber with the position of the optical path and the position of the fiber axis shifted.
Therefore, because the position of incidence light to an optical filter changed, it was necessary to adjust the position of an optical filter in this optical module. Especially, in the case that the optical module has a high performance optical filter such as the narrowband band-pass filters, the highly accurate tuning in optical characteristic is indispensable.
Generally as the tuning method in optical characteristic of an optical module, there are known the first method of adjusting by changing the slope angle of an optical filter to the optical path, and the second method of adjusting by changing the position of an optical filter in vertical plane or in predetermined incident angle to the optical path.
However, in a reflection type optical module for instance, an optical fiber module and an optical wavelength multiplexing/de-multiplexing module (WDM module), the position where the reflected optical signal is coupled with formation changes according to slope angle of the slope surface when an optical characteristic is adjusted by the first method.
Therefore, there is a problem that it is necessary to adjust the position of the optical fiber, which couple the reflected optical signal whenever the slope angle of the slope surface is adjusted, thus assembly becomes difficult in a reflection type optical module.
Moreover, the change of the slope angle increases the insertion loss for ferrule, which has a plurality of optical fibers arranged at predetermined intervals.
Therefore, it is necessary to fix the slope angle so as to be a predetermined angle, and the tuning of an optical characteristic by the angle adjustment is impossible in the optical module having the above ferrule.
On the other hand, because the slope angle is maintained, the tuning method in optical characteristic of an optical module by the second method can be applied when an optical module has a plurality of optical fibers.
However, the angle adjustment function of the optical filter is needed for an optical module of the reflection type to unite the optical signal that can be reflected from the optical filter with the optical fiber on the output side. In other words, it is difficult to unite the optical signal that can be reflected from the optical filter with the optical fiber only by the mechanical accuracy of parts that compose the optical module.
Moreover, after the angle of the optical filter is adjusted, to achieve the function to fix adjusted angle and the function to fix the optical filter changed in a vertical plane to the optical path at the same time, the compositional parts increase extremely and the adjustment becomes difficult. Therefore, it is difficult to manufacture an optical module having high accuracy and high reliability at low cost.
The present invention is a method for tuning optical characteristic of an optical module comprising of an optical fiber, a lens, an optical filter and a filter holder, comprising: a step of arranging said optical fiber, said lens, said optical filter fixed said filter holder, a step of tuning an optical characteristic to turn the optical filter or the said optical fiber around the axis offset from the center of said optical filter.